My component diagrams are able to show that the Travel system supports both JSP (web) and Rich GUI clients. Do I have to create TWO sequence diagrams (one for web clients, one for GUI clients) for each use case? Example, 2 use case diagrams for Prepare Itinerary use case.
You can present sequence diagrams in client neutral manner. You can just present the business tier interactions all requeset originating from common client. However, that depends on your design, If your design has common business tier for both client types you can use the above technique.